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United States Patent |
5,668,847
|
Hernandez
|
September 16, 1997
|
Apparatus and method for adjusting radiation in a radiation-emitting
device
Abstract
In a radiation emitting device, particularly in a radiation treatment
device, the actual radiation delivered to an object via a radiation beam
is adjusted dependent on the dimensions of an opening in a plate
arrangement provided between a radiation source and an object so that the
radiation output has a constant wedge factor over an irradiation field,
regardless of the size of the opening. The wedge factor is defined as the
ratio between a reference radiation output along a reference axis of the
beam with a predetermined physical wedge in the beam path and an actual
radiation output of the beam in a substantiallylossless beam path.
Inventors:
|
Hernandez; Francisco M. (Concord, CA)
|
Assignee:
|
Siemens Medical Systems, Inc. (Iselin, NJ)
|
Appl. No.:
|
665153 |
Filed:
|
June 13, 1996 |
Current U.S. Class: |
378/65; 250/492.1; 378/150 |
Intern'l Class: |
G21F 005/04 |
Field of Search: |
378/65,108,145,147,156,151,152,159,150
250/492.1,492.3,505.1
|
References Cited
U.S. Patent Documents
5148032 | Sep., 1992 | Hernandez | 250/492.
|
5563925 | Oct., 1996 | Hernandez | 378/150.
|
Other References
Faiz M. Khan, Ph.D., "The Physics of Radiation Therapy", 2d ed., pp.
200-206 no date.
Siemens Product Brochure, "Digital Systems for Radiation Oncology", pp.
1-16 no date.
|
Primary Examiner: Porta; David P.
Attorney, Agent or Firm: Vance; Heather S.
Parent Case Text
This is a continuation, of application Ser. No. 08/504,937 filed Jul. 20,
1995 now U.S. Pat. No. 5,563,925.
Claims
I claim:
1. A method for adjusting radiation output delivered to an object from a
radiation source, the method comprising the steps of:
generating a radiation beam having a variable radiation output from a
radiation source to the object;
delimiting the beam path by moving at least one beam-shielding device;
defining a field on the object for irradiation;
calculating a wedge correction signal, the wedge correction signal
indicating how to change the variable radiation output to a desired dose
distribution for the area on the object, the desired dose distribution
having a wedge factor; and
varying the radiation output of the radiation beam with the beam-shielding
device, the varying being directly related to the wedge correction signal
such that the wedge factor for the radiation output is constant regardless
of the size of the irradiated field.
2. The method for adjusting radiation output delivered to an object from a
radiation source of claim 1, wherein the wedge correction signal is an
additive offset.
3. The method for adjusting radiation output delivered to an object from a
radiation source of claim 2, wherein a circuit adds the wedge correction
signal to at least one nominal dose signal.
4. The method for adjusting radiation output delivered to an object from a
radiation source of claim 1, wherein the wedge correction signal is stored
in a memory, and wherein a program uses the stored wedge correction signal
to vary the radiation output of the radiation beam.
5. The method for adjusting radiation output delivered to an object from a
radiation source of claim 1, wherein the wedge correction signal is a
wedge correction factor.
6. The method for adjusting radiation output delivered to an object from a
radiation source of claim 5, wherein the wedge correction factor is a
multiplier.
7. The method for adjusting radiation output delivered to an object from a
radiation source of claim 5, wherein parameters of the wedge correction
factor are stored for various field sizes.
8. The method for adjusting radiation output delivered to an object from a
radiation source of claim 5, wherein the wedge correction factor is
associated with at least one of the beam-shielding devices.
9. A system for adjusting radiation output delivered to an object from a
radiation source, comprising:
a radiation source generating a radiation beam having a variable radiation
output;
identifying means for defining a field on the object for irradiation;
beam shielding means for delimiting the variable radiation output from the
radiation beam to at least one defined irradiation field on the object;
a dose controller for varying a degree of shielding of the radiation beam
such that a desired dose distribution is obtained the desired dose
distribution having a wedge factor; and means for calculating a correction
signal, and for using the correction signal for varying the radiation
output such that the wedge factor is constant regardless of the size of
the irradiation field.
10. The system for adjusting radiation output delivered to an object from a
radiation source of claim 9, wherein the correction signal is obtained
from a plurality of additive offsets.
11. The system for adjusting radiation output delivered to an object from a
radiation source of claim 9, wherein the correction signal is obtained
from plurality of multipliers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a radiation emitting device, and particularly to a
system and a method for adjusting the radiation delivered to an object in
a radiation treatment device.
2. Description of the Related Art
Radiation-emitting devices are generally known and used, for instance as
radiation therapy devices for the treatment of patients. A radiation
therapy device generally comprises a gantry which can be swiveled around a
horizontal axis of rotation in the course of a therapeutic treatment. A
linear accelerator is located in the gantry for generating a high-energy
radiation beam for therapy. This high energy radiation beam can be an
electron radiation or photon (X-ray) beam. During treatment, this
radiation beam is trained on one zone of a patient lying in the isocenter
of the gantry rotation.
In order to control the radiation emitted toward an object, a
beam-shielding device such as a plate arrangement or collimator is usually
provided in the trajectory of the radiation beam between the radiation
source and the object. This beam-shielding device defines a field on the
object to which a prescribed amount of radiation is to be delivered.
The radiation delivered to an object may be analyzed into primary and
scattered components. The primary radiation is made up of the initial or
original photons emitted from the radiation source, and the scattered
radiation is the result of the photons scattered by the plate arrangement
itself. The beam's radiation output in free space increases because of the
increased collimator scatter, which is added to the primary beam. In other
words, a point in the field is subjected not only to direct radiation,
that is the primary component, but also to radiation that is scattered
from the plate arrangement. The ratio of the radiation output in air with
the scatterer to the radiation output without the scatterer for a
reference field (for instance 10.times.10 cm) is commonly called the
"output factor" or the collimator scatter factor. The concept and
definition of the output factor are well understood in the art.
Thus, due to these scattered photons, the dose rate applied to the surface
of the object changes dependent on the size of the opening in the plate
arrangement, that is, on the field size. This means that the radiation
emitted to the same spot, for instance in the center of the radiation beam
onto the object, changes according to the size of the opening in the plate
arrangement. When the plate arrangement shows only a small opening, then
the accumulated dose at the same spot is less than the accumulated dose at
the same spot when the opening is big.
Frequently, special filters or absorbing blocks are located in the
trajectory of the radiation beam to modify its isodose distribution. A
most commonly used filter is the wedge filter. This is a wedge-shaped
absorbing block which causes a progressive decrease in the intensity
across the beam, resulting in isodose curves that are modified relative to
their normal positions. Such wedge filters are usually made of dense
material, such as lead or steel, or other absorbing material.
The presence of a wedge filter decreases the output of the
radiation-emitting device and this decrease must be taken into account in
treatment calculations. This effect is characterized by the so-called
"wedge factor", defined as the ratio of doses with and without the wedge
at a point in the object along the central axis of the radiation beam. The
wedge factor depends on the material, size and angle of the wedge. Wedges,
and particular the wedge factor, are described in Faiz M. Khan, Ph.D, "The
Physics of Radiation Therapy", Williams & Wilkins, pages 234 to 238.
The delivery of radiation by such a radiation therapy device is prescribed
and approved by an oncologist. Actual operation of the radiation
equipment, however, is normally done by a therapist. When the therapist
administers the actual delivery of the radiation treatment as prescribed
by the oncologist, the device is programmed to deliver that specific
treatment. When programming the treatment, the therapist has to take into
consideration the output factor and has to adjust the dose delivery based
on the plate arrangement opening in order to achieve the prescribed
radiation output on the surface of the object. This adjustment can be made
according to known calculations, but the therapist normally has to do them
manually, which can easily lead to errors. In the context of radiation
therapy, a miscalculation can lead to either a dose that is too low and is
ineffective, or that is too high and dangerous; a large error, for
example, a misplaced decimal point, can be lethal.
U.S. Pat. No. 5,148,032 discloses a radiation treatment device in which
isodose curves in the object are adjusted both by a plate arrangement,
which includes at least one movable plate that is controlled during
irradiation, and by varying the radiation output of the radiation beam
during irradiation, so that a wide range of variations in the possible
isodose curves is obtained. A wedge-shaped isodose distribution is
established, for example, by moving one plate at a constant speed while
simultaneously changing the radiation output of the radiation beam. In
this radiation treatment device there is no physical absorbing block in
the trajectory of the radiation beam, and the therapist has to take this
into account.
What is needed is a method, and corresponding system, for adjusting the
delivery of radiation to the object in order to make sure that the
actually delivered radiation output is exactly the same as the desired
radiation output, independent of the use of a wedge function.
SUMMARY OF THE INVENTION
According to the invention, radiation output delivered to an object from a
radiation source is adjusted by generating a radiation beam having a
variable radiation output and a substantially lossless beam path from a
radiation source to the object. The beam path is delimited by moving at
least one beam-shielding device such as a movable plate. An irradiated
field of the object is defined. The radiation output of the beam is varied
as a predetermined function of the position of the beam-shielding device,
a wedge factor of the radiation output thereby varying according to a
predetermined profile, in which the wedge factor is defined as the ratio
between a reference radiation output along a reference axis of the beam
with a predetermined physical wedge in the beam path and an actual
radiation output of the beam in a substantially lossless beam path. The
radiation output is varied such that the wedge factor is constant
regardless of the size of the irradiated field, and is preferably equal to
unity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a radiation treatment device including
a treatment console constructed in accordance with the present invention.
FIG. 2 is a block diagram illustrating portions of a processing unit, a
control unit and a beam generation system in the radiation treatment
device of. FIG. 1.
FIG. 3 shows isodose curves for a conventional wedge filter built of an
absorption block in the path of a radiation beam.
FIG. 4 shows isodose curves for an arrangement in which a wedge filter is
realized by the movement of one plate of a plate arrangement in the path
of the radiation beam and by changing the radiation output of the
radiation beam.
DETAILED DESCRIPTION
The invention is described below with primary reference to a system for
delivering X-ray radiation to a field of a patient, and for delimiting the
field using at least one movable plate in the beam path from a radiation
source. This is by way of example only. The invention may be used to
adjust the delivery of any type of energy, for example, electrons (instead
of X-rays), to any type of object (not just a human patient), provided the
amount of energy delivered to the field can be sensed or estimated.
FIG. 1 shows a radiation treatment device 2 of common design, in which
plates 4 and a control unit in a housing 9 and a treatment unit 100
constructed in accordance with the principles of the invention are used.
The radiation treatment device 2 comprises a gantry 6 which can be
swiveled around a horizontal axis of rotation 8 in the course of a
therapeutic treatment. Plates 4 are fastened to a projection of gantry 6.
To generate the high-powered radiation required for the therapy, a linear
accelerator is located in gantry 6. The axis of the radiation bundle
emitted from the linear accelerator and gantry 6 is designated by 10.
Electron, photon, or any other detectable radiation can be used for the
therapy.
During the treatment the radiation beam is trained on a zone 12 of an
object 13, for example, a patient who is to be treated, and who lies at
the isocenter of the gantry rotation. The rotational axis 8 of gantry 6,
the rotational axis 14 of a treatment table 16, and the beam axis 10 all
preferably intersect in the isocenter. The construction of such a
radiation treatment device is described in general in a brochure "Digital
Systems for Radiation Oncology", Siemens Medical Laboratories, Inc.
A91004-M2630-B358-01-4A00, September 1991.
The area of the patient that is irradiated is known as the field. As is
well known, the plates 4 are substantially impervious to the emitted
radiation. They are mounted between the radiation source and the patient
in order to delimit the field. Areas of the body, for example, healthy
tissue, are therefore subjected to as little radiation as possible, and
preferably to none at all. In the preferred embodiment of the invention,
at least one of the plates is movable so that the distribution of
radiation over the field need not be uniform (one region can be given a
higher dose than another); furthermore the gantry can preferably be
rotated so as to allow different beam angles and radiation distributions
without having to move the patient around. Neither or these features is
necessary according to the invention: the invention may also be used with
fixed-field devices (no movable plates), with constant radiation delivery
rates, and with fixed-angle beams (no rotatable gantry).
Radiation treatment device 2 also includes a central treatment processing
or control unit 100, which is usually located apart from radiation
treatment device 2. The radiation treatment device 2 is normally located
in a different room to protect the therapist from radiation. Treatment
unit 100 includes output devices, such as at least one visual display unit
or monitor 70, and an input device such as a keyboard 19, although data
can be input also through data carriers, such as data storage devices, or
an verification and recording or automatic set-up system 102, which is
described below. The treatment processing unit 100 is typically operated
by the therapist who administers actual delivery of a radiation treatment
as prescribed by an oncologist. By utilizing the keyboard 19, or other
input device, the therapist enters into a control unit 76 of the treatment
unit 100 the data that defines the radiation to be delivered to the
patient, for example, according to the prescription of the oncologist. The
program can also be input via another input device like a data storage
device, through data transmission, or using the automatic set-up system
102. On the screen of a monitor 70 various data can be displayed before
and during the treatment.
FIG. 2 shows portions of an illustrative radiation treatment device 2 and
portions of treatment unit 100 in more detail. An electron beam 1 is
generated in an electron accelerator 20. Accelerator 20 comprises an
electron gun 21, a wave guide 22 and an evacuated envelope or guide magnet
23. A trigger system 3 generates injector trigger signals and supplies
them to injector 5. Based on these injector trigger signals, injector 5
generates injector pulses which are fed to electron gun 21 in accelerator
20 for generating electron beam 1. Electron beam 1 is accelerated and
guided by wave guide 22. For this purpose, a high frequency (HF) source
(not shown) is provided which supplies radio frequency (RF) signals for
the generation of an electromagnetic field supplied to wave guide 22. The
electrons injected by injector 5 and emitted by electron gun 21 are
accelerated by this electromagnetic field in wave guide 22 and exit at the
end opposite to electron gun 21 as electron beam 1. Electron beam 1 then
enters a guide magnet 23, and from there is guided through a window 7
along axis 10. After passing through a first scattering foil 15, the beam
goes through a passageway 51 of a shield block 50 and encounters a second
scattering foil 17. Next, it is sent through a measuring chamber 60, in
which the dose is ascertained. If the scattering foils are replaced by a
target, the radiation beam is an X-ray beam. Finally, the aperture plate
arrangement 4 is provided in the path of radiation beam 1, by which the
irradiated field of the subject of investigation is determined. Aperture
plate arrangement 4 includes a pair of plates 41 and 42. As is described
above, this is just one example of a beam-shielding arrangement that can
be used in the invention. The invention will work with others also as long
as there is an aperture plate arrangement that defines an irradiated
field.
Plate arrangement 4 comprises a pair of aperture plates 41 and 42 and an
additional pair of aperture plates (not shown) arranged perpendicular to
plates 41 and 42. In order to change the size of the irradiated field the
aperture plate can be moved with respect to axis 10 by a drive unit 43
which is indicated in FIG. 2 only with respect to plate 41. Drive unit 43
comprises an electric motor which is coupled to plates 41 and 42 and which
is controlled by a motor controller 40. Position sensors 44 and 45 are
also coupled to plates 41 and 42, respectively, for sensing their
positions. This is just one example of such a system. The invention will
work with other systems also, as long as them is a beam-shielding
arrangement that defines an irradiated field and as long as sensors are
provided to indicate the field size.
Motor controller 40 is coupled to a dose control unit 61 which includes a
dosimetry controller and which is coupled to a central processing unit 18
for providing set values for the radiation beam for achieving given
isodose curves. The output of the radiation beam is measured by a
measuring chamber 60. In response to the deviation between the set values
and the actual values, dose control unit 61 supplies signals to trigger
system 3, which changes in a known manner the pulse repetition frequency
so that the deviation between the set values and the actual values of the
radiation beam output is minimized.
In such a radiation treatment device the dose absorbed by object 13 is
dependent on the type of filter used for shaping the radiation beam. If a
wedge filter built from absorbing material is inserted in the trajectory
of the radiation beam, then the preset dose has to be increased according
to the wedge factor in order to supply the desired dose to object 13.
FIG. 3 shows isodose curves for a conventional wedge filter 46 in the path
of the radiation beam emitted from radiation source 17 to object 13. The
radiation beam is shaped on the one hand by the wedge filter and on the
other hand by aperture plates 41 and 42. Due to the absorbing material of
wedge filter 46, the isodose curve in the center 10 of the beam on object
13 has a maximum value of Dmax, which is the maximum value at a spot in
center 10 of the beam on the surface of object 13 without wedge filter 46.
In the illustrated example, Dmax is roughly 72%. The wedge factor defined
as the ratio of doses with and without wedge filter 46 is thus, in this
case 0.72.
FIG. 4 shows isodose curves in a radiation treatment device according to
the invention. Instead of including a wedge-shaped absorber in the path of
the radiation beam, the filter function is performed by changing the
radiation output of the radiation beam and by simultaneously moving at
least one plate 41 and keeping the other plates of plate arrangement 4
stationary. A radiation treatment device having such a filter arrangement
is disclosed in U.S. Pat. No. 5,148,032. Although this U.S. Patent
describes the possibility of moving any plate, in the following, the
invention is described in connection with only one plate being moved and
the other plates being kept stationary. This is for the sake of simplicity
only. The invention may be used for multiple moving plates as well.
When in FIG. 4 plate 41 moves in the direction of arrow A toward plate 42
and at the same time the radiation output is changed according to a
desired wedge angle, by adjusting the speed of plate 41 and/or
correspondingly, the value of the isodose curve through the center of the
beam on the surface of object 13 equals Dmax=100%. Thus, by using a wedge
function instead of a wedge-shaped absorber an efficiency factor of "1" or
100% can be established; in other words, the dose delivered at that point
is 100% of the prescribed dose, although the same relative isodose
profiles are maintained. That means that the therapist does not have to
take into account a wedge factor when defining the treatment, although
wedge shaped isodose curves are established.
FIG. 2 shows those portions of treatment unit 100 which are necessary to
carry out the invention. Treatment unit 100 comprises central processing
unit 18 and which is programmed by the therapist according to the
instructions of the oncologist so that the radiation treatment device
carries out the prescribed radiation treatment. Through keyboard 19 the
prescribed delivery of the radiation treatment is input.
Central processing unit 18 is connected, on the one hand, with the input
device, such as the keyboard 19, for inputting the prescribed delivery of
the radiation treatment and, on the other hand, with a dose control unit
61 that generates the desired values of radiation for the controlling
trigger system 3. Trigger system 3 then adapts the pulse repetition
frequency or other parameters in a corresponding, conventional manner. The
ability to change the radiation output is generally known and it is
particularly advantageous to use a digital dosimetry system became then it
can easily be controlled by the digital output of central processing unit
18.
Central processing unit 18 includes control unit 76 which controls the
execution of the program and which supplies position signals P for
controlling the opening of plate arrangement 4 and nominal dose signals D
(corresponding to the plate position that would be demanded using prior
art methods, that is, without regard to output factor compensation) for
adjusting the radiation output at the output of radiation source 17. A
memory 77 is also provided in or is connected to the central processing
unit 18 for supplying correction signals C, which the processing unit uses
to adjust the radiation output dependent on the position signals P
supplied from position sensors 44 and 45 in order to achieve the
predetermined constant output factor.
The preferred arrangement of the memory unit is that, for each plate
position (field size), it has stored a corresponding wedge correction
signal C. The memory thus stores a table of wedge correction factors. If
more than one set of movable plates is included in the system, then the
table will be correspondingly multi-dimensional, and arranged using any
known data structure, so that a wedge correction factor is available for
any combination of plate positions.
Control unit 76 and memory 77 apply the nominal dose and wedge correction
signals D and C, respectively, to a combination circuit 78, which combines
the values to generate set signals S. The set signals S are in turn
applied to the dose control unit 61, which sets the radiation output.
The combination circuit 78 will depend on the form in which the wedge
correction signals are generated and stored. Assume that the wedge
correction signals C are stored as additive offsets to the set radiation
output. In this case, the combination circuit will be an adder which adds
the wedge correction signals C to nominal dose signals D. This is the
preferred embodiment, since it is simplest. If, however, the wedge
correction factors are multipliers, for example, an increase in radiation
output from a sensed value of 72% would require a multiplicative
correction signal of about 139%. Instead of storing actual values of the
wedge correction signals C, it is also possible to store the parameters of
a wedge correction function for the various field sizes. The processing
unit would then evaluate the wedge correction function for each current
field size using the parameters stored in the memory, and would then
generate the wedge correction signals (additive or multiplicative) itself.
The wedge correction signals are determined before actual treatment of a
patient in one or more calibration runs. To determine relative wedge
correction values, a reference surface is irradiated with a known
reference plate position, and the radiation output over the surface is
sensed by a conventional sensing device (not shown), which generates
radiation output signals, which are applied to the processing unit 18. In
particular, the radiation output at a reference point (for example, at the
center of the beam) is sensed. The reference surface need not lie in the
patient plane, although if it does the calibration will typically be
easier and more accurate.
The plates are then moved to a new opening position, the radiation output
is sensed and the needed amount of adjustment is determined to create the
proper isodose profile for that position. This process is continued until
correction values are stored for the reference surface over the entire
range of motion of the plates. If more than one set of movable plates is
included, then correction values will be sensed and stored for each
combination of plate positions; the number of combinations will depend on
the desired or required resolution.
The correction values indicate by how much the radiation output (for
example, dose rate) is to be changed (via the wedge correction signals)
such that the delivered dose distribution is equal to the desired dose
distribution, that is, the isodose profiles are generated corresponding to
what they would be if the radiation output were held constant and a
physical wedge were included in the beam path. During actual treatment,
for each plate position, the processing unit adjusts the radiation output
to correspond to what is needed to generate the correct isodose profile.
Since no actual physical wedge is included, however, and the system is
calibrated for 100% output at the reference point, the therapist need not
perform any calculations to adjust for a wedge factor. If additive offsets
are chosen for the wedge correction factors, then the difference between
the sensed output values and the desired output value is stored. If
multiplicative correction factors are chosen, then ratios are stored.
Alternatively, any known function approximation method may be used to
generate the parameters of an approximating function of the additive or
multiplicative wedge correction factors required.
A "course" of radiation treatment may, and often does, have more than one
field, and may run over several different sessions. In some cases,
hundreds of different (and, in some cases, fixed) sequential fields with
different wedges are used during a course, for example, to provide proper
irradiation of a field that has a complicated geometry or prescribed dose
profile, to lessen discomfort to the patient, or to adjust the field as a
tumor shrinks during treatment. The invention therefore also comprises an
optional verification and recording or "auto set-up" system 102 (see FIG.
2), which stores and downloads to the radiation system (via the CPU 18 or
directly into the memory) the parameters, for example, of the geometry, of
the various fields of the course of treatment, and/or the tables of wedge
correction factors that were derived during earlier calibration runs for
the various fields.
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